/* See COPYRIGHT for copyright information. */

#include <inc/x86.h>
#include <inc/mmu.h>
#include <inc/error.h>
#include <inc/string.h>
#include <inc/assert.h>

#include <kern/pmap.h>
#include <kern/kclock.h>

// These variables are set by i386_detect_memory()
static physaddr_t maxpa;	// Maximum physical address
size_t npage;				// Amount of physical memory (in pages)
static size_t basemem;		// Amount of base memory (in bytes)
static size_t extmem;		// Amount of extended memory (in bytes)

// These variables are set in i386_vm_init()
pde_t* boot_pgdir;			// Virtual address of boot time page directory
physaddr_t boot_cr3;		// Physical address of boot time page directory
static char* boot_freemem;	// Pointer to next byte of free mem

struct Page* pages;		// Virtual address of physical page array
static struct Page_list page_free_list;	// Free list of physical pages

// Global descriptor table.
// The kernel and user segments are identical(except for the DPL).
// To load the SS register, the CPL must equal the DPL.  Thus,
// we must duplicate the segments for the user and the kernel.
struct Segdesc gdt[] =
{
	// 0x0 - unused (always faults -- for trapping NULL far pointers)
	SEG_NULL,

	// 0x8 - kernel code segment
	[GD_KT >> 3] = SEG(STA_X | STA_R, 0x0, 0xffffffff, 0),

	// 0x10 - kernel data segment
	[GD_KD >> 3] = SEG(STA_W, 0x0, 0xffffffff, 0),

	// 0x18 - user code segment
	[GD_UT >> 3] = SEG(STA_X | STA_R, 0x0, 0xffffffff, 3),

	// 0x20 - user data segment
	[GD_UD >> 3] = SEG(STA_W, 0x0, 0xffffffff, 3),

	// 0x28 - tss, initialized in idt_init()
	[GD_TSS >> 3] = SEG_NULL
};

struct Pseudodesc gdt_pd =
{
	0, sizeof(gdt) - 1, (unsigned long) gdt,
};

static int
nvram_read(int r)
{
	return mc146818_read(r) | (mc146818_read(r + 1) << 8);
}

void
i386_detect_memory(void)
{
	// CMOS tells us how many kilobytes there are
	basemem = ROUNDDOWN(nvram_read(NVRAM_BASELO)*1024, PGSIZE);
	extmem = ROUNDDOWN(nvram_read(NVRAM_EXTLO)*1024, PGSIZE);

	// Calculate the maxmium physical address based on whether
	// or not there is any extended memory.  See comment in ../inc/mmu.h.
	if (extmem)
		maxpa = EXTPHYSMEM + extmem;
	else
		maxpa = basemem;

	npage = maxpa / PGSIZE;

	cprintf("Physical memory: %dK available, ", (int)(maxpa/1024));
	cprintf("base = %dK, extended = %dK\n", (int)(basemem/1024), (int)(extmem/1024));
}

// --------------------------------------------------------------
// Set up initial memory mappings and turn on MMU.
// --------------------------------------------------------------

static void check_boot_pgdir(void);

// Allocate n bytes of physical memory aligned on an 
// align-byte boundary.  Align must be a power of two.
// Return kernel virtual address.  Returned memory is uninitialized.

// If we're out of memory, boot_alloc should panic.
// It's too early to run out of memory.
// This function may ONLY be used during initialization,
// before the page_free_list has been set up.

static void*
boot_alloc(uint32_t n, uint32_t align)
{
	extern char end[];
	void *v;

	// Initialize boot_freemem if this is the first time.
	// 'end' is a magic symbol automatically generated by the linker,
	// which points to the end of the kernel's bss segment -
	// i.e., the first virtual address that the linker
	// did _not_ assign to any kernel code or global variables.
	if (boot_freemem == 0) boot_freemem = end;

	// Your code here:
	//	Step 1: round boot_freemem up to be aligned properly
	//	Step 2: save current value of boot_freemem as allocated chunk
	//	Step 3: increase boot_freemem to record allocation
	//	Step 4: return allocated chunk

	boot_freemem = ROUNDUP(boot_freemem, align);
	v = (void*)boot_freemem;
	boot_freemem += n;
	return v;
}

// Given pgdir, a pointer to a page directory,
// walk the 2-level page table structure to find
// the page table entry (PTE) for linear address la.
// Return a pointer to this PTE.

// If the relevant page table doesn't exist in the page directory:
//	- If create == 0, return 0.
//	- Otherwise allocate a new page table, install it into pgdir,
//	  and return a pointer into it.
//        (Questions: What data should the new page table contain?
//	  And what permissions should the new pgdir entry have?
//	  Note that we use the 486-only "WP" feature of %cr0, which
//	  affects the way supervisor-mode writes are checked.)

// This function abstracts away the 2-level nature of
// the page directory by allocating new page tables
// as needed.

// Boot_pgdir_walk cannot fail.  It's too early to fail.
// This function may ONLY be used during initialization,
// before the page_free_list has been set up.

static pte_t*
boot_pgdir_walk(pde_t *pgdir, uintptr_t la, int create)
{
	//Entry correspondiente la direccion lineal
	pde_t *pde = &pgdir[PDX(la)];

	//Si la tabla de paginas esta presente
	if (*pde & PTE_P)
		//Devuelve un puntero a la tabla
		return (pte_t*)KADDR(PTE_ADDR(*pde)) + PTX(la);
	else
	{
		//Si no hay table hay que crearla
		if (create)
		{
			pte_t *pt = (pte_t*)boot_alloc(PGSIZE, PGSIZE);
			memset(pt, 0, PGSIZE);
			void *ppt = (void*)PADDR(pt);
			*pde = PTE_ADDR(ppt)|PTE_W|PTE_P;
			return (pt + PTX(la));
		}
		else
			return (pte_t*)0;
	}
}

// Map [la, la+size) of linear address space to physical [pa, pa+size)
// in the page table rooted at pgdir.  Size is a multiple of PGSIZE.
// Use permission bits perm|PTE_P for the entries.

// This function may ONLY be used during initialization,
// before the page_free_list has been set up.
static void
boot_map_segment(pde_t *pgdir, uintptr_t la, size_t size, physaddr_t pa, int perm)
{
	int i;
	
	for (i = 0; i < size / PGSIZE; i++)
	{
		pte_t *pte = boot_pgdir_walk(pgdir, la + i * PGSIZE, 1);
		*pte = PTE_ADDR(pa + i * PGSIZE)|perm|PTE_P;
	}
}

// Set up a two-level page table:
//    boot_pgdir is its linear (virtual) address of the root
//    boot_cr3 is the physical adresss of the root
// Then turn on paging.  Then effectively turn off segmentation.
// (i.e., the segment base addrs are set to zero).

// This function only sets up the kernel part of the address space
// (ie. addresses >= UTOP).  The user part of the address space
// will be setup later.

// From UTOP to ULIM, the user is allowed to read but not write.
// Above ULIM the user cannot read (or write). 
void
i386_vm_init(void)
{
	pde_t* pgdir;
	uint32_t cr0;
	//size_t n;

	// Remove this line when you're ready to test this function.
	//~ panic("i386_vm_init: This function is not finished\n");

	//////////////////////////////////////////////////////////////////////
	// create initial page directory.
	pgdir = boot_alloc(PGSIZE, PGSIZE);
	memset(pgdir, 0, PGSIZE);
	boot_pgdir = pgdir;
	boot_cr3 = PADDR(pgdir);

	//////////////////////////////////////////////////////////////////////
	// Recursively insert PD in itself as a page table, to form
	// a virtual page table at virtual address VPT.
	// (For now, you don't have understand the greater purpose of the
	// following two lines.)

	// Permissions: kernel RW, user NONE
	pgdir[PDX(VPT)] = PADDR(pgdir)|PTE_W|PTE_P;

	// same for UVPT
	// Permissions: kernel R, user R 
	pgdir[PDX(UVPT)] = PADDR(pgdir)|PTE_U|PTE_P;

	//////////////////////////////////////////////////////////////////////
	// Map the kernel stack (symbol name "bootstack").  The complete VA
	// range of the stack, [KSTACKTOP-PTSIZE, KSTACKTOP), breaks into two
	// pieces:
	//     * [KSTACKTOP-KSTKSIZE, KSTACKTOP) -- backed by physical memory
	//     * [KSTACKTOP-PTSIZE, KSTACKTOP-KSTKSIZE) -- not backed => faults
	//     Permissions: kernel RW, user NONE
	// Your code goes here:

	boot_map_segment(pgdir, KSTACKTOP-KSTKSIZE, KSTKSIZE, PADDR(bootstack), PTE_W); 

	//////////////////////////////////////////////////////////////////////
	// Map all of physical memory at KERNBASE. 
	// Ie.  the VA range [KERNBASE, 2^32) should map to
	//      the PA range [0, 2^32 - KERNBASE)
	// We might not have 2^32 - KERNBASE bytes of physical memory, but
	// we just set up the mapping anyway.
	// Permissions: kernel RW, user NONE
	// Your code goes here:

	boot_map_segment(pgdir, KERNBASE, 0xFFFFFFFF - KERNBASE + 1, 0, PTE_W);

	//////////////////////////////////////////////////////////////////////
	// Make 'pages' point to an array of size 'npage' of 'struct Page'.
	// The kernel uses this structure to keep track of physical pages;
	// 'npage' equals the number of physical pages in memory.  User-level
	// programs get read-only access to the array as well.
	// You must allocate the array yourself.
	// Map this array read-only by the user at linear address UPAGES
	// (ie. perm = PTE_U | PTE_P)
	// Permissions:
	//    - pages -- kernel RW, user NONE
	//    - the image mapped at UPAGES  -- kernel R, user R
	// Your code goes here: 

	size_t spages = ROUNDUP(npage * sizeof(struct Page), PGSIZE);
	pages = (struct Page*)boot_alloc(spages, PGSIZE);
	physaddr_t ppages = PADDR(pages);
	boot_map_segment(pgdir, UPAGES, spages, ppages, PTE_U);
	
	// Check that the initial page directory has been set up correctly.
	check_boot_pgdir();

	//////////////////////////////////////////////////////////////////////
	// On x86, segmentation maps a VA to a LA (linear addr) and
	// paging maps the LA to a PA.  I.e. VA => LA => PA.  If paging is
	// turned off the LA is used as the PA.  Note: there is no way to
	// turn off segmentation.  The closest thing is to set the base
	// address to 0, so the VA => LA mapping is the identity.

	// Current mapping: VA KERNBASE+x => PA x.
	//     (segmentation base=-KERNBASE and paging is off)

	// From here on down we must maintain this VA KERNBASE + x => PA x
	// mapping, even though we are turning on paging and reconfiguring
	// segmentation.

	// Map VA 0:4MB same as VA KERNBASE, i.e. to PA 0:4MB.
	// (Limits our kernel to <4MB)
	pgdir[0] = pgdir[PDX(KERNBASE)];

	// Install page table.
	lcr3(boot_cr3);

	// Turn on paging.
	cr0 = rcr0();
	cr0 |= CR0_PE|CR0_PG|CR0_AM|CR0_WP|CR0_NE|CR0_TS|CR0_EM|CR0_MP;
	cr0 &= ~(CR0_TS|CR0_EM);
	lcr0(cr0);

	// Current mapping: KERNBASE+x => x => x.
	// (x < 4MB so uses paging pgdir[0])

	// Reload all segment registers.
	asm volatile("lgdt gdt_pd+2");
	asm volatile("movw %%ax,%%gs" :: "a" (GD_UD|3));
	asm volatile("movw %%ax,%%fs" :: "a" (GD_UD|3));
	asm volatile("movw %%ax,%%es" :: "a" (GD_KD));
	asm volatile("movw %%ax,%%ds" :: "a" (GD_KD));
	asm volatile("movw %%ax,%%ss" :: "a" (GD_KD));
	asm volatile("ljmp %0,$1f\n 1:\n" :: "i" (GD_KT));  // reload cs
	asm volatile("lldt %%ax" :: "a" (0));

	// Final mapping: KERNBASE+x => KERNBASE+x => x.

	// This mapping was only used after paging was turned on but
	// before the segment registers were reloaded.
	pgdir[0] = 0;

	// Flush the TLB for good measure, to kill the pgdir[0] mapping.
	lcr3(boot_cr3);
}

// Checks that the kernel part of virtual address space
// has been setup roughly correctly(by i386_vm_init()).

// This function doesn't test every corner case,
// in fact it doesn't test the permission bits at all,
// but it is a pretty good sanity check. 

static physaddr_t check_va2pa(pde_t *pgdir, uintptr_t va);

static void
check_boot_pgdir(void)
{
	cprintf("check_boot_pgdir() begins!\n");
	uint32_t i, n;
	pde_t *pgdir;

	pgdir = boot_pgdir;

	// check pages array
	n = ROUNDUP(npage*sizeof(struct Page), PGSIZE);
	for (i = 0; i < n; i += PGSIZE)
		assert(check_va2pa(pgdir, UPAGES + i) == PADDR(pages) + i);

	// check phys mem
	for (i = 0; KERNBASE + i != 0; i += PGSIZE)
		assert(check_va2pa(pgdir, KERNBASE + i) == i);

	// check kernel stack
	for (i = 0; i < KSTKSIZE; i += PGSIZE)
		assert(check_va2pa(pgdir, KSTACKTOP - KSTKSIZE + i) == PADDR(bootstack) + i);

	// check for zero/non-zero in PDEs
	for (i = 0; i < NPDENTRIES; i++) {
		switch (i) {
		case PDX(VPT):
		case PDX(UVPT):
		case PDX(KSTACKTOP-1):
		case PDX(UPAGES):
			assert(pgdir[i]);
			break;
		default:
			if (i >= PDX(KERNBASE))
				assert(pgdir[i]);
			else
				assert(pgdir[i] == 0);
			break;
		}
	}
	cprintf("check_boot_pgdir() succeeded!\n");
}

// This function returns the physical address of the page containing 'va',
// defined by the page directory 'pgdir'.  The hardware normally performs
// this functionality for us!  We define our own version to help check
// the check_boot_pgdir() function; it shouldn't be used elsewhere.

static physaddr_t
check_va2pa(pde_t *pgdir, uintptr_t va)
{
	pte_t *p;

	pgdir = &pgdir[PDX(va)];
	if (!(*pgdir & PTE_P))
		return ~0;
	p = (pte_t*) KADDR(PTE_ADDR(*pgdir));
	if (!(p[PTX(va)] & PTE_P))
		return ~0;
	return PTE_ADDR(p[PTX(va)]);
}

// --------------------------------------------------------------
// Tracking of physical pages.
// The 'pages' array has one 'struct Page' entry per physical page.
// Pages are reference counted, and free pages are kept on a linked list.
// --------------------------------------------------------------

// Initialize page structure and memory free list.
// After this point, ONLY use the functions below
// to allocate and deallocate physical memory via the page_free_list,
// and NEVER use boot_alloc() or the related boot-time functions above.

void
page_init(void)
{
	// The exaple code here marks all pages as free.
	// However this is not truly the case.  What memory is free?
	//  1) Mark page 0 as in use.
	//     This way we preserve the real-mode IDT and BIOS structures
	//     in case we ever need them.  (Currently we don't, but...)
	//  2) Mark the rest of base memory as free.
	//  3) Then comes the IO hole [IOPHYSMEM, EXTPHYSMEM).
	//     Mark it as in use so that it can never be allocated.      
	//  4) Then extended memory [EXTPHYSMEM, ...).
	//     Some of it is in use, some is free. Where is the kernel?
	//     Which pages are used for page tables and other data structures?

	int i;
	extern char end[];
	LIST_INIT(&page_free_list);

	size_t nbasepages = IOPHYSMEM / PGSIZE;
	size_t ntotalbasepages = EXTPHYSMEM / PGSIZE;

	physaddr_t kernbase =  PADDR(0xF0100000);
	physaddr_t structend = PADDR(boot_freemem);
	
	for (i = 0; i < npage; i++)
	{
		if (i == 0)
			// First page is in use
			pages[i].pp_ref = 1;

		else if (i >= nbasepages && i < ntotalbasepages)
			// IO pages are in use
			pages[i].pp_ref = 1;

		else if (i >= (kernbase / 1024) && i < (structend / PGSIZE))
			// Kernel code and data structures are in use
			pages[i].pp_ref = 1;

		else {
			// The rest of the pages are free
			pages[i].pp_ref = 0;
			LIST_INSERT_HEAD(&page_free_list, &pages[i], pp_link);
		}
	}
}


// Initialize a Page structure.
// The result has null links and 0 refcount.
// Note that the corresponding physical page is NOT initialized!

static void
page_initpp(struct Page *pp)
{
	memset(pp, 0, sizeof(*pp));
}

// Allocates a physical page.
// Does NOT set the contents of the physical page to zero -
// the caller must do that if necessary.

// *pp -- is set to point to the Page struct of the newly allocated page

// RETURNS 
//   0 -- on success
//   -E_NO_MEM -- otherwise 

int
page_alloc(struct Page **pp_store)
{
	if(!LIST_EMPTY(&page_free_list))
	{
		*pp_store = LIST_FIRST(&page_free_list);
		LIST_REMOVE(*pp_store, pp_link);
		page_initpp(*pp_store);
		return 0;
	}

	return -E_NO_MEM;
}

// Return a page to the free list.
// (This function should only be called when pp->pp_ref reaches 0.)
void
page_free(struct Page *pp)
{
	LIST_INSERT_HEAD(&page_free_list, pp, pp_link);
}

// Decrement the reference count on a page, freeing it if there are no more refs.
void
page_decref(struct Page* pp)
{
	if (--pp->pp_ref == 0) page_free(pp);
}

// This is boot_pgdir_walk with a different allocate function:
// namely, it should use page_alloc() instead of boot_alloc().
// Unlike boot_pgdir_walk, pgdir_walk can fail, so we have to
// return pte via a pointer parameter.

// Stores address of page table entry in *ppte.
// Stores 0 if there is no such entry or on error.

// RETURNS: 
//   0 on success
//   -E_NO_MEM, if page table couldn't be allocated
int
pgdir_walk(pde_t *pgdir, const void *va, int create, pte_t **pte_store)
{
    pgdir = &pgdir[PDX(va)];
    if(!(*pgdir & PTE_P))
    {
        if(!create)
        {
        	*pte_store = 0;
        	return 0;
        }

	    struct Page *pp;
        //Si no puedo allocar la pagina tiro error
        if(page_alloc(&pp) != 0) return -E_NO_MEM;

        //Si pude aumento la cantidad de referencias
        pp->pp_ref++;
        physaddr_t physicalAddress = page2pa(pp);
        memset(KADDR(PTE_ADDR(physicalAddress)), 0 ,PGSIZE);
        *pgdir = physicalAddress | PTE_U | PTE_P | PTE_W;
    }

    pte_t *entry = (pte_t*) KADDR(PTE_ADDR((*pgdir)));
    *pte_store = &entry[PTX(va)];
    return 0;
}

// Map the physical page 'pp' at virtual address 'va'.
// The permissions (the low 12 bits) of the page table
//  entry should be set to 'perm|PTE_P'.

// Details
//   - If there is already a page mapped at 'va', it is page_remove()d.
//   - If necesary, on demand, allocates a page table and inserts it into 'pgdir'.
//   - pp->pp_ref should be incremented if the insertion succeeds

// RETURNS: 
//   0 on success
//   -E_NO_MEM, if page table couldn't be allocated

int
page_insert(pde_t *pgdir, struct Page *pp, void *va, int perm)
{
    pte_t *entry;
    //Obtengo la entrada en la tabla de paginas para la direccion virtual
    if(pgdir_walk(pgdir, va, 1, &entry) != 0) return -E_NO_MEM;
    
    //Apunta a una pagina presente?
    if(*entry & PTE_P)
    {
        //La presente no es igual a la que quiero insertar
        if(pa2page(PTE_ADDR(*entry)) != pp)
        {
            page_remove(pgdir, va);
            pp->pp_ref++;
        }
        *entry = page2pa(pp) | perm | PTE_P;    
    }
    else
    {
        *entry = page2pa(pp) | perm | PTE_P;
        pp->pp_ref++;
    }

    return 0;
}

// Return the page mapped at virtual address 'va'.
// If ppte is not zero, then we store in it the address
// of the pte for this page.  This is used by page_remove
// but should not be used by other callers.
// Return 0 if there is no page mapped at va.
struct Page *
page_lookup(pde_t *pgdir, void *va, pte_t **pte_store)
{
    pgdir_walk(pgdir, va, 0 , pte_store);
    if((*pte_store != 0) && (**pte_store & PTE_P))
		return pa2page(PTE_ADDR(**pte_store));

    return 0;
}

// Unmaps the physical page at virtual address 'va'.
// Details:
//   - The ref count on the physical page should decrement.
//   - The physical page should be freed if the refcount reaches 0.
//   - The pg table entry corresponding to 'va' should be set to 0.
//     (if such a PTE exists)
//   - The TLB must be invalidated if you remove an entry from
//	   the pg dir/pg table.
void
page_remove(pde_t *pgdir, void *va)
{
	pte_t *entry;
	struct Page *page = page_lookup(pgdir, va, &entry);
	if(!page) return;

	//Decremento la cantidad de referencias a la pagina
	page_decref(page);

	//Seteo la entrada en la tabla como 0
	if(entry) *entry = 0;

	//Invalido la TLB dado que cambie los valores
	tlb_invalidate(pgdir, va);
}

// Invalidate a TLB entry, but only if the page tables being
// edited are the ones currently in use by the processor.
void
tlb_invalidate(pde_t *pgdir, void *va)
{
	invlpg(va);
}

void
page_check(void)
{
	cprintf("page_check() begins!\n");
	
	struct Page *pp, *pp0, *pp1, *pp2;
	struct Page_list fl;

	// should be able to allocate three pages
	pp0 = pp1 = pp2 = 0;
	assert(page_alloc(&pp0) == 0);
	assert(page_alloc(&pp1) == 0);
	assert(page_alloc(&pp2) == 0);

	assert(pp0);
	assert(pp1 && pp1 != pp0);
	assert(pp2 && pp2 != pp1 && pp2 != pp0);

	// temporarily steal the rest of the free pages
	fl = page_free_list;
	LIST_INIT(&page_free_list);

	// should be no free memory
	assert(page_alloc(&pp) == -E_NO_MEM);

	// there is no free memory, so we can't allocate a page table 
	assert(page_insert(boot_pgdir, pp1, 0x0, 0) < 0);

	// free pp0 and try again: pp0 should be used for page table
	page_free(pp0);
	assert(page_insert(boot_pgdir, pp1, 0x0, 0) == 0);
	assert(PTE_ADDR(boot_pgdir[0]) == page2pa(pp0));
	assert(check_va2pa(boot_pgdir, 0x0) == page2pa(pp1));
	assert(pp1->pp_ref == 1);
	assert(pp0->pp_ref == 1);

	// should be able to map pp2 at PGSIZE because pp0 is already allocated for page table
	assert(page_insert(boot_pgdir, pp2, (void*) PGSIZE, 0) == 0);
	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp2));
	assert(pp2->pp_ref == 1);

	// should be no free memory
	assert(page_alloc(&pp) == -E_NO_MEM);

	// should be able to map pp2 at PGSIZE because it's already there
	assert(page_insert(boot_pgdir, pp2, (void*) PGSIZE, 0) == 0);
	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp2));
	assert(pp2->pp_ref == 1);

	// pp2 should NOT be on the free list
	// could happen in ref counts are handled sloppily in page_insert
	assert(page_alloc(&pp) == -E_NO_MEM);

	// should not be able to map at PTSIZE because need free page for page table
	assert(page_insert(boot_pgdir, pp0, (void*) PTSIZE, 0) < 0);

	// insert pp1 at PGSIZE (replacing pp2)
	assert(page_insert(boot_pgdir, pp1, (void*) PGSIZE, 0) == 0);

	// should have pp1 at both 0 and PGSIZE, pp2 nowhere, ...
	assert(check_va2pa(boot_pgdir, 0) == page2pa(pp1));
	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp1));
	// ... and ref counts should reflect this
	assert(pp1->pp_ref == 2);
	assert(pp2->pp_ref == 0);

	// pp2 should be returned by page_alloc
	assert(page_alloc(&pp) == 0 && pp == pp2);

	// unmapping pp1 at 0 should keep pp1 at PGSIZE
	page_remove(boot_pgdir, 0x0);
	assert(check_va2pa(boot_pgdir, 0x0) == ~0);
	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp1));
	assert(pp1->pp_ref == 1);
	assert(pp2->pp_ref == 0);

	// unmapping pp1 at PGSIZE should free it
	page_remove(boot_pgdir, (void*) PGSIZE);
	assert(check_va2pa(boot_pgdir, 0x0) == ~0);
	assert(check_va2pa(boot_pgdir, PGSIZE) == ~0);
	assert(pp1->pp_ref == 0);
	assert(pp2->pp_ref == 0);

	// so it should be returned by page_alloc
	assert(page_alloc(&pp) == 0 && pp == pp1);

	// should be no free memory
	assert(page_alloc(&pp) == -E_NO_MEM);

	// forcibly take pp0 back
	assert(PTE_ADDR(boot_pgdir[0]) == page2pa(pp0));
	boot_pgdir[0] = 0;
	assert(pp0->pp_ref == 1);
	pp0->pp_ref = 0;

	// give free list back
	page_free_list = fl;

	// free the pages we took
	page_free(pp0);
	page_free(pp1);
	page_free(pp2);

	cprintf("page_check() succeeded!\n");
}

